Characterization of the Gut Bacteria involved in Isoflavone and Estrogen Metabolism

The conversion of the soy isoflavone daidzein to equol, through gut bacterial metabolism, is associated with reduced breast cancer risk, although only 30 to 50% of humans excrete equol. In-vitro studies with equol have shown that it can inhibit enzymes involved in steroid biotransformation suggesting that the relationship between equol and circulating estrogens may be due to equol’s effect on hormone metabolism. Alternatively, estrogen metabolism can differ depending on the species of gut bacteria and the administration of antibiotics has been shown to change circulating estrogen levels. Recent studies in our lab have shown that there was a significant difference in estrogen metabolism by gut bacteria from equol producing and non-producing study participants. Therefore, the composition of the microbial community may mediate changes in the estrogenicity of the circulating estrogen pool and account for some of the association between breast cancer risk and equol producer status. However, we do not know which members of the gut microbial community that produce equol are responsible for altering the estrogenicity of circulating estrogens.

In this study we will characterize the gut bacterial community structure associated with differences in estrogen metabolism between equol producing (EP) and equol non-producing (ENP) individuals. Identification of the members of the gut microbial community involved in equol production and estrogen metabolism will help elucidate whether they play a role in estrogen metabolism and can influence the composition and estrogenicity of circulating estrogens as related to breast cancer risk.